If you don't want to divest yourself of your 3*s, spend the cores to expand your storage capacity. I'm keeping one of every tank, and going so far as to boost each one up to the max 30% XP bonus, just because this is a waifu collection game after all and they've even begun expanding the stories to 3* tanks. While they're easy enough to replace if they suddenly gain a use, it isn't really hurting me that much to keep them on hand. Spending the cores is preventing me from opening the fifth factory slot by, what...a few days at most?

BTW, regarding Ilona's stories, the patch notes said they were 'wholesome'...WTF? Lolis aren't really my thing, but this is still an H-game and they shouldn't even bother if they're not going to do it the right way.